From my experience it has a lot to do with where you get hit. If you take a dozen hits to the face, you still move around, dazed, but standing. Take a well aimed blow to the chin or a clean shot to the temple and bang, good night. Size doesn't realy matter with knockouts. But you can sure as hell take more punches to safe zones if you have a more robust head. The chin is a nerve plexus, just like the solar plexus. Doesn't matter how strong you are, how big your head is. A shock to the nerves takes out the trash. Whereas the temple is the thinest bone of the skull, massive trauma there (kicks, elbows) is almost identical to hitting the brain itself. The reaction is almost instantenous lights out. These shocks can range from falling to the ground like a rock just to hop on your feet 5 seconds later, to complete and total loss of consciousness.
Of course this is only the applied science part and ppl are welcome to disagree, but if you want hard proof just browse around for videos. Karate, mma, tae-kwon-do, boxing, and observe the k.o. Unless the dude collapses from fatigue its always a hit to the temple, the chin or the solar plexus.